The present invention relates to a method and a system for reading a radiation image that has been stored in a photostimulable phosphor screen.
Radiation image recording systems wherein a radiation image is recorded on a photostimulable phosphor screen by exposing the screen to image-wise modulated penetrating radiation are widely used nowadays.
The recorded image is reproduced by stimulating the exposed photostimulable phosphor screen by means of stimulating radiation and by detecting the light that is emitted by the phosphor screen upon stimulation and converting the detected light into an electrical signal representation of the radiation image.
In several applications e.g. in mammography, sharpness of the image is a very critical parameter.
Sharpness of an image that has been read out of a photostimulable phosphor screen depends not only on the sharpness and resolution of the screen itself but also on the resolution obtained by the read out system which is used.
In conventional read out systems used nowadays a scanning unit of the flying spot type is commonly used. Such a scanning unit comprises a source of stimulating radiation, e.g. a laser light source, means for deflecting light emitted by the laser so as to form a scanning line on the photostimulable phosphor screen and optical means for focussing the laser beam onto the screen.
Examples of such systems are the Agfa Diagnostic Systems, denominated by the trade name ADC 70 and Agfa Compact. In these systems photostimulable phosphor screens are commonly used which comprise a BaFBr:Eu phosphor.
The resolution of the read out apparatus is mainly determined by the spot size of the laser beam. This spot size in its turn depends on the characteristics of the optical light focussing arrangement.
It has been recognised that optimizing the resolution of a scanning system may result in loss of optical collection efficiency of the focussing optics. As a consequence an important fraction of the laser light is not focussed onto the image screen.
A severe prejudice exists against the use of systems having an optical collection efficiency of the focussing optics which is less than 50% because these systems were expected not to deliver an adequate amount of power to the screen in order to read out this screen to a sufficient extent within an acceptable scanning time.
For example in the case of a BaFBr:Eu phosphor a minimal power value of 15 mW on the screen is required to read the image to a sufficient extent within a reasonable time.
Even when all the light emitted by the laser would be captured by the focussing optics, the optical collection efficiency would still not be higher than 50% because of reflections at the optical elements and because of losses at mirrors that are used in the system.
Applying a scanning system that only captures 50% or less of the laser light reduces the collection efficiency to 25% and hence would require the use of a laser which is stronger than 60 mW.
This requirement needs to be complemented by the requirement to have a small emitting area.
Both requirements are hard to solve by means of an inexpensive solution.
It is an object of the present invention to provide a method and a system for reading a radiation image that has been stored in a photostimulable phosphor screen.
It is a further object of the invention to provide such a method and system that yields a high sharpness.
The above mentioned objects are realised by a method according to claim 1.
Another aspect of the invention relates to an apparatus as set out in claim 6.
Specific features for preferred embodiments of the invention are set out in the dependent claims.
According to the present invention a screen comprising a divalent europium activated cesium halide phosphor wherein said halide is at least one of chloride and bromide is used. The laser beam which is used to stimulate the screen is focussed so that the spot diameter of the laser spot emitted by that laser, measured between 1/e2 points of the gaussian profile of the laser beam, is smaller than 100 micrometer, preferably even smaller than 50 micrometer.
The present invention provides a solution to the prior art problems described in the introductory part of the present invention.
The use of the divalent europium activated cesium halide phosphor allows the use of focussing optics with a low collection efficiency.
In this way the scanning unit can be optimized with regard to small laser spot size on the phosphor screen so that a high resolution system is obtained.
Because the divalent europium activated cesium halide phosphor inherently has a very good optical stimulablility, occasional loss of optical efficiency which originates from the measures taken to obtain a small laser spot size, becomes acceptable. The occasional loss of optical efficiency does not deteriorate the overall image quality obtained in the radiation image read out system, nor does it have a negative effect on the overall throughput of the system.
The photostimulable phosphor screen applied in the present invention comprises a divalent europium activated cesium halide phosphor wherein said halide is at least one of chloride and bromide.
Such a phosphor is known in the art and has for example been disclosed in EP-A-174 875 (and U.S. Pat. No. 5,028,509). The phosphor is especially well suited for manufacturing xe2x80x98binderlessxe2x80x99 phosphor screens. Binderless phosphor screens provide optimal sharpness.
It is advantageous however to use a CsX:Eu phosphor wherein X represents a halide selected from the group consisting of Br and Cl which is obtained by the following method:
mixing CsX with between 10xe2x88x923 and 5 mol % of a Europium compound selected from the group consisting of EuXxe2x80x22, EuXxe2x80x23 and EuOXxe2x80x2, Xxe2x80x2 being a member selected from the group consisting of F, Cl, Br and I,
firing the mixture at a temperature above 450xc2x0 C.
cooling said mixture and
recovering the CsX:Eu phosphor.
A phosphor that has been obtained as a result of the above method of preparation has an increased conversion efficiency compared to the state of the art divalent europium activated cesium halide phosphor. The phosphor can be stimulated by means of a lower amount of stimulating light energy.
A photostimulable phosphor screen using such a phosphor is preferably obtained by the method of
preparing said CsX:Eu phosphor by firing a mixture of said CsX with between 10-3 and 5 mol % of an Europium compound selected from the group consisting of EuXxe2x80x22, EuXxe2x80x23 and EuOXxe2x80x2, Xxe2x80x2 being a halide selected from the group consisting of F, Cl, Br and I and
applying said phosphor on a substrate by a method selected from the group consisting of physical vapor deposition, thermal vapor deposition, chemical vapor deposition, radio frequency deposition and pulsed laser deposition.
This method of preparation is advantageous because it allows to deposit the phosphor in the form of needle-shaped crystals. These needle-shaped phosphor crystals act as light guides so that they reduce the lateral spreading of light in the phosphor layer. Reduced lateral light spread leads to images of higher resolution.
Alternatively a phosphor screen containing a CsX:Eu stimulable phosphor, wherein X represents a halide selected from the group consisting of Br and Cl can also be manufactured by performing the steps of:
bringing multiple containers of said CsX and an Europium compound selected from the group consisting of EuXxe2x80x22, EuXxe2x80x23 and EuOXxe2x80x2, Xxe2x80x2 being a halide selected from the group consisting of F, Cl, Br and I in condition for vapor deposition and
depositing, by a method selected from the group consisting of physical vapor deposition, thermal vapor deposition, chemical vapor deposition, electron beam deposition, radio frequency deposition and pulsed laser deposition, both said CsX and said Europium compound on a substrate in such a ratio that on said substrate a CsX phosphor, doped with between 10xe2x88x923 and 5 mol % of an Europium compound, is formed.
This method of preparation is advantageous because it likewise allows to deposit the phosphor in the form of needle-shaped crystals. These needle-shaped phosphor crystals act as light guides so that they reduce the lateral spreading of light in the phosphor layer. Reduced lateral light spread leads to images of higher resolution.
The above phosphors and screen preparation methods have been described in the following U.S. provisional applications which are incorporated by reference into the present application: Nos. 60/159,004 and 60/142,276.
Further advantages and embodiments of the present invention will become apparent from the following description and drawings.